The present invention relates to an optical pickup of an optical device capable of recording information on magneto-optical disc and reproducing the information.
There are optical discs such as a write once (WORM) disc and a magneto-optical disc. These discs are different from the CD in the material of the recording surface.
For example, a WORM disc has a tellurium or bismuth recording surface on which the lasers burn pits for recording. In another type of WORM disks, the lasers are focused on a recording surface coated with a selenium antimony (Sb.sub.2 Se.sub.3) thin film, or an oxide tellurium (TeOx) thin film, or a thin film of organic pigment, changing the reflectivity of the light.
The magneto-optical disc uses as the recording surface, an amorphous alloy made of rare earth metals such as gallium, terbium, and others. In a magneto-optical recording method, the recording surface of the disc is initially magnetized to form a magnetic field in a direction perpendicular to the surface. The laser heats a predetermined area of the disc to elevate the temperature above Curie temperature, which is about 150.degree. C., thereby reversing the direction of the magnetic field. To read the recorded information, the laser is irradiated on the recording surface so that polarized wave front slightly rotates as a result of the Kerr effect. Thus only the polarized wave deflected by the rotation is read by a photodetector, thereby enabling to read the information.
In order to read the recorded information, a focus servo system and a tracking servo system are provided for maintaining the distance between the recording surface and a pickup constant.
FIG. 5 schematically shows a conventional optical system of a pickup in a magneto-optical reproducing system. Laser reflected from a magneto-optical disc (not shown) enters a prism 2 through a convex lens 1. The prism 2 has a triangular portion 2a, a rectangular plate portion 2b, and a polarizing film 2c interposed between the portions 2a and 2b. The polarizing film 2c allows only the P(parallel)-polarization component of the reflected light to pass through. Therefore, the S(senkrecht)-polarization component of the reflected light passing through the plate portion 2a is reflected at a reflecting surface thereof and applied to a photodetector 3. On the other hand, the P-polarization component passes through the polarizing film 2c and the rectangular plate portion 2b, and is reflected at a rear surface of the plate portion 2b, and applied to the detector 3.
Referring to FIG. 6, the detector 3 has two detecting areas PD1 and PD2 for receiving spots of light of the S-polarization component and the P-polarization component, respectively. The areas PD1 and PD2 are divided into three sections to generate outputs A, B, and C, and A', B', and C', respectively.
A focus error signal F.sub.E is obtained using a spot size method, wherein sizes of spots formed by the S-polarization component and P-polarization component are compared with each other. As shown in FIG. 7, the focus error signal F.sub.E is calculated as follows. EQU F.sub.E =(A+C-B)-(A'+C'-B')
When the spot sizes formed on the respective detecting areas PD1 and PD2 are equal to each other, the error signal F.sub.E is zero. This means that the laser is appropriately focused on the disc. FIG. 8 shows such a state, where a focus F of the lens 1 is properly located. When the spot size of the S-polarization component on the area PD1 is larger than that of the P-polarization component formed on the are a PD2, the error signal F.sub.E is larger than zero. FIG. 9 shows such a state, where the focus F is far away from the prism 2. Namely, an objective is too close to the recording surface.
To the contrary, if the spot size formed by the S-polarization component is smaller than the spot size formed by the P-polarization component than the spot size formed by the P-polarization component as shown in FIG. 10, F.sub.E is smaller than zero. Hence, the focus F is close to the prism 2, indicating that the objective is too far from the disc. Therefore, the objective must be moved to the disk.
Thus, the focus servo system operates to maintain the appropriate position of the objective where the spot sizes-on the detecting areas PD1 and PD2 become equal.
FIG. 11 shows an example of a calculating circuit for producing an RF signal which represents information retrieved from the optical disc. In the example, the data signal RF is obtained by calculating a difference between the S-polarization component and the P-polarization component detected by the detecting areas PD1 and PD2, respectively, as follows. EQU RF=(A+B+C)-(A'+B'+C')
In the optical system using the spot size method, the dynamic range of the system can be improved by increasing the spot sizes. This can be attained by increasing the thickness of the rectangular plate portion 2b of the prism 2. Namely, as shown in FIGS. 12 and 13, when the thickness t is increased, the distance between focuses F1 and F2 increases, so that the spots are enlarged from d1 to d2. Thus, the dynamic range is increased. However, the increase of the thickness of the plate 2b also causes a distance D between the S-polarization component and P-polarization component applied to the detector 3 to increase to D1. Accordingly the distance between the detecting areas PD1 and PD2 must be increased, so that the detector 3, and hence the optical pickup becomes large.